Method and device for starting a drive train

ABSTRACT

In a method and a drive for starting a drive train, with a drive shaft ( 2 ), a drive motor ( 4 ) connected to an electrical grid ( 12 ) and with a differential gearing ( 3 ) with three drives and outputs, wherein one output is connected to the drive shaft ( 2 ), a first drive is connected to the drive motor ( 4 ) and a second drive is connected to a differential drive ( 5 ), the drive motor ( 4 ) is started from a rotational speed of zero or approximately zero while an external braking torque acts on the drive shaft ( 2 ), and the second drive is braked in an acceleration phase of the drive shaft ( 2 ).

The invention relates to a method for starting a drive train with a drive shaft, a prime mover, and with a differential gear system with three inputs and outputs, whereby one output is connected to the drive shaft, a first input is connected to the prime mover, and a second input is connected to a differential drive.

The invention relates, furthermore, to an input for implementing this method.

One general problem of driven machines, such as delivery systems, e.g., pumps, compressors and fans, or such as mills, crushers, vehicles, etc., is efficient variable-speed operation, or starting under high load, since, e.g., electrical machines, but also internal combustion engines, in most cases have a lower starting torque than their rated torque. In addition, electrical machines are used as the example for prime movers, but the principle applies to all possible types of prime movers, such as, e.g., internal combustion engines.

The most frequently used electrical inputs are currently three-phase machines, such as, e.g., asynchronous motors and synchronous motors. In spite of high electrical power consumption, three-phase machines at rest are not able to deliver this power fully mechanically; this is reflected in high losses and a low starting torque. At the same time, the current consumption of a three-phase machine when starting from speed zero typically corresponds to roughly 7 times the rated current; this causes a correspondingly high electrical load for the grid during starting.

Therefore, a three-phase machine must be designed to be correspondingly large so that it can deliver an input torque that corresponds to the rated torque from rest and is for this reason often oversized. Also for this reason, electrical machines are therefore often designed in combination with a frequency converter as a variable-speed input instead of being connected directly to a grid. Thus, starting with high torque from speed zero can be implemented without loading the grid; the approach is, however, expensive and associated with major losses of efficiency. One alternative that is more cost-favorable and also better with respect to efficiency compared to this is the use of differential systems—for example according to AT 507394 A. The fundamental limitation here, however, is that depending on the transmission ratio of the differential stage, only a relatively small speed range or, in the so-called differential mode, essentially no low speeds can be achieved on the drive shaft of a driven machine.

There are various possibilities for doing this. According to German Utility Model DE 20 2012 101 708 U, for example, the transmission ratio of the differential gear system can be fixed at 1. On this basis, the complete drive train can be driven with the differential drive or the prime mover can be brought to synchronous speed, and the latter can subsequently synchronize with the grid.

The disadvantage of this approach is that the differential drive or its frequency converter is significantly smaller-sized than the prime mover and therefore can also only deliver a correspondingly small torque.

The object of the invention is therefore to find a solution with which prime movers under load can either be synchronized with the grid (such as, e.g., electrical machines coupled directly to the grid) or can be accelerated into a speed range with high available torque (such as, e.g., in internal combustion engines), and in addition, the driven machine can be started up from speed zero with optimal design torque of the drive train.

This object is achieved with a method of the above-mentioned type in such a way that the prime mover is started up from a speed of zero or approximately zero, while an external braking torque acts on the drive shaft and in such a way that in an acceleration phase of the drive shaft, the second drive is braked.

This object is achieved in addition with an input with the features of claim 13.

The heart of a differential system is a differential gear system, which in a simple embodiment can be a simple planetary gear stage with three inputs and outputs, whereby one output is connected to the drive shaft of a driven machine, a first input is connected to the prime mover, and a second input is connected to a differential drive. Thus, the driven machine can be operated at variable speeds when the prime mover is at constant speed by the differential drive compensating for the difference in speed.

In order to bring a prime mover from a standstill preferably up to synchronous speed and in addition to start up a driven machine with high torque from speed zero, the start-up can take place according to the invention, e.g., as follows in 3 phases:

Phase 1: The prime mover is preferably switched to the grid using a so-called star/delta connection or alternatively (in an especially grid-friendly method) first using an additional system, it is brought to (at least roughly) synchronous speed and then synchronized with the grid. In the case of an internal combustion engine, the latter is simply started and then accelerated. In doing so, the prime mover remains largely free of external mechanical loads during starting, aside from the reaction forces of the second input of the differential gear system, which forces are caused by the mass moment of inertia and must be overcome. By implication, this means that until the prime mover has reached its rated speed, a correspondingly small driving torque acts on the drive shaft of the driven machine.

Phase 2: Since the full torque of the prime mover is now available, in the second phase, the actual acceleration and starting of the driven machine begin under load by the second input of the differential gear stage being decelerated by means of a synchronization brake.

Phase 3: As soon as the drive shaft of the second input of the differential system is in the governed speed range of the differential drive, the latter takes over the speed control of the drive train and the synchronization brake is released.

Preferred embodiments of the invention are the subject of the subclaims.

Below, preferred embodiments of the invention are explained with reference to the attached drawings. Here:

FIG. 1 shows the principle of a differential system according to the invention for one input of a pump,

FIG. 2 shows another embodiment according to the invention of a differential system,

FIG. 3 shows another embodiment according to the invention of a differential system with a gear driving stage,

FIG. 4 shows the speed parameters and power parameters of a differential system of a pump,

FIG. 5 shows another embodiment according to the invention of a differential system with a simplified differential drive,

FIG. 6 shows the speed parameters and power parameters following from FIG. 5,

FIG. 7 shows another embodiment according to the invention of a differential system with a gear shifting stage,

FIG. 8 shows the speed parameters and power parameters following from FIG. 7,

FIG. 9 shows another embodiment according to the invention of a differential system with a reduced speed range,

FIG. 10 shows the speed parameters and power parameters following from FIG. 9,

FIG. 11 shows the possible speed parameters and power parameters resulting from FIG. 9 for a so-called pump turbine,

FIG. 12 shows another embodiment according to the invention of a differential system for an internal combustion engine as a prime mover, and

FIG. 13 shows a control system for damping drive train vibrations.

FIG. 1 shows the principle of a differential system for a drive train in the example of a pump. Here, the driven machine 1 is the rotor of a pump, which is driven via a drive shaft 2 and a differential gear system 3 from a prime mover 4. The prime mover 4 is preferably a medium-voltage three-phase machine that is connected to a grid 12 that in the illustrated example is a medium-voltage grid based on a medium-voltage three-phase machine. The chosen voltage level depends, however, on the application and mainly the power level of the prime mover 4 and can have any desired voltage level without influencing the basic function of the system according to the invention. According to the number of pole pairs of the prime mover 4, a design-specific operating speed range is produced. Here, the operating speed range is that speed range in which the prime mover 4 can deliver a defined or desired or necessary torque and in the case of an electrical prime mover can be synchronized with the grid 12. A planetary carrier 7 is connected to the drive shaft 2, the prime mover 4 is connected to a ring gear 8, and a sun wheel 9 of the differential gear system 3 is connected to a differential drive 5. The heart of the differential system in this embodiment is thus a simple planetary gear stage with three inputs and outputs, whereby one output is connected to the drive shaft 2 of the driven machine 1, a first input is connected to the prime mover 4, and a second input is connected to the differential drive 5.

In order to be able to optimally adapt the speed range of the differential drive 5, an adjusting gear system 10 is implemented between the sun wheel 9 and the differential drive 5. Alternatively to the illustrated spur wheel stage, the adjusting gear system 10 can, for example, also be made multi-stage or as a toothed belt or chain drive and/or can be combined with a planetary gearing stage. Moreover, with the adjusting gear system 10, an axial offset for the differential drive 5 can be implemented that due to the coaxial arrangement of the driven machine 1 and the prime mover 4 makes possible a simple design of the differential drive 5. A motor brake 13 that if necessary brakes the differential drive 5 is connected to the differential drive 5. Electrically, the differential drive 5 is linked to the grid 12 by means of a preferably low-voltage frequency converter, consisting of a motor-side inverter 6 a and a grid-side inverter 6 b, and a transformer 11. The transformer equalizes any existing voltage differences between the grid 12 and the grid-side inverter 6 b, and can be omitted at voltage equality between the prime mover 4, the grid-side inverter 6 b, and the grid 12. The inverters 6 a and 6 b are connected by a d.c. intermediate circuit and if necessary can be separated locally, whereby preferably the motor-side inverter 6 a is positioned as near as possible at the differential drive 5. The major advantage of this concept is that the prime mover 4 can be linked directly, i.e., without complex power electronics, to a grid 12. The equalization between variable rotor speed and fixed speed of the grid-linked prime mover 4 is implemented by the variable-speed differential drive 5.

The torque equation for the differential system is:

Torque_(differential drive)=Torque_(drive shaft) *y/x,

whereby the size factor y/x is a measure of the transmission ratios in the differential gear system 3 and in the adjusting gear system 10. The power of the differential drive 5 is essentially proportional to the product of the percentage deviation of the pump speed from its base speed×drive shaft output. Accordingly, a large speed range in principle requires a correspondingly large dimensioning of the differential drive 5. This is also the reason why differential systems are especially well suited for small speed ranges, but in principle any speed range is achievable.

A differential drive 5 for a pump as a driven machine 1 has, for example, a power of roughly 15% of the total system power. This in turn means that low speeds on the driven machine 1 cannot be implemented with the differential system. If the driven machine 1 must be brought from speed zero with high torque into its operating speed range (this is the speed range in which the driven machine 1 essentially operates), this can only be implemented by the differential drive 5 being braked (either electrically or by means of a motor brake 13) and the prime mover 4 being switched to the grid. The prime mover 4 in turn can only apply the rated torque with difficulty from rest or it draws up to 7 times the rated current in order to accelerate roughly to synchronous speed. By using a so-called star/delta connection, the starting current can be reduced, but thus also the attainable starting torque is reduced.

An improvement according to the invention is achieved, e.g., by the differential drive 5 at the beginning of starting being brought to its maximum possible operating speed. Due to external loads, in the meantime, the driven machine 1 remains in a range of low speed. In this way, the prime mover 4 is brought to a speed that is necessarily established depending on the speed of the driven machine 1, on the one hand, and the transmission ratio of the differential gear system 3 and an optionally present adjusting gear system 10, on the other hand. Then, the differential drive 5 is regulated such that its speed remains within its governed speed range, while the prime mover 4 is switched to the grid 12 with or without a so-called star/delta connection. The speed regulation or braking of the differential drive 5 in this case is carried out preferably electrically by the inverter 6 a, 6 b or by means of a motor brake 13.

The motor brake 13 can also be used to protect the differential drive 5 from overspeeds, when, e.g., the prime mover 4 fails and the driven machine 1 stops or turns in the opposite direction.

FIG. 2 shows another embodiment according to the invention of a differential system. Here, the illustrated drive train as in FIG. 1 also has a driven machine 1, a drive shaft 2, a differential gear system 3, a prime mover 4, and a differential drive 5 that is connected to the grid 12 by means of a frequency converter 6 (consisting of motor-side and grid-side inverters—here shown simplified as a unit) and a transformer 11. Here, the differential drive 5 is also linked to the differential gear system 3 by means of an adjusting gear system 10. In addition, however, a clutch 15 is implemented between the adjusting gear system 10 and the differential gear system 3.

A synchronization brake 14 acts on the sun wheel 9 and thus on the entire drive train. When starting, in this embodiment of the invention in a first step, the differential drive 5 and the adjusting gear system 10 are decoupled by the clutch 15 from the remainder of the drive train. If the prime mover 4 is now accelerated and connected to the grid, the sun wheel 9 turns freely at the same time, and no noteworthy torque can build up in the entire drive train. Thus, in this case, the driven machine 1 also remains in a range of low speed, and the prime mover 4 can be synchronized with the grid 12 without noteworthy external counter-torque.

In order to avoid the above-described effect of the high starting current when the prime mover 4 is being synchronized, either a star/delta connection can be implemented or the prime mover 4 can be brought to (approximately) synchronous speed by an auxiliary system—e.g., a small variable-speed input—and then can be synchronized with the grid 12. Alternatively, with the clutch 15 engaged—as already described in FIG. 1—the prime mover 4 can be brought to speed with the differential drive 5. In doing so, the prime mover 4 cannot be accelerated to its synchronous speed, but at least the starting current that is being established is smaller. The clutch 15 is then disengaged again.

An alternative method for smooth grid synchronization of the electrical machine 4 would in this case be to separate, on the one hand, the frequency converter 6 from the differential drive 5 and, on the other hand, the electrical machine 4 from the grid 12. Subsequently, the electrical machine 4 can be synchronized with the grid 12 by means of the frequency converter 6, then the electrical machine 4 connecting to the grid 12, and after that the frequency converter 6 can be connected (again) to the differential drive 5. Thus, the electrical machine 4 can be switched smoothly to the grid 12. In this case, the differential drive 5 would begin with the variable-speed regulation of the drive train only as soon as the drive shaft of the differential gear system 3 connected to the sun wheel 9 lies in the governed speed range of the differential drive 5.

As soon as the prime mover 4 accelerates above a certain speed and the driven machine 1 in the meantime is turning only slowly, on the sun wheel 9, a speed is established that is high according to the transmission ratio of the differential gear system 3 and that (with consideration of the adjusting gear system 10) is above the allowed governed speed range for the differential drive 5. The governed speed range is the speed range in which the differential drive 5 works in order to be able to implement the operating speed range of the driven machine 1. The governed speed range is determined in doing so mainly by the voltage limits, current limits and speed limits that have been specified by the manufacturer. In this phase, the differential drive 5 cannot be connected to the grid 12. In another step, therefore, the second input of the differential gear system 3, which input is connected to the sun wheel 9, is decelerated with the synchronization brake 14 to a speed that is in the governed speed range of the differential drive 5. This can, depending on the braking system produced or the requirements of the drive train, be done both with and without regulation in terms of speed/torque. Subsequently, the differential-drive-side part of the clutch 15 is preferably synchronized (preferably by means of the differential drive 5) with the speed of the second input of the differential gear system 3, and then the clutch 15 is engaged. The clutch 15 is preferably a positive jaw clutch or a non-positive multi-disk clutch. One advantage of the non-positive multi-disk clutch is that if it is designed for this purpose, synchronization of the two clutch halves is not necessary. The clutch 15 can be omitted when the differential drive 5 is designed for the speeds established during the start-up process. Thus, the motor brake 13 can subsequently replace the synchronization brake 14.

In order to be able to achieve high torque, which lies above the torque of the differential drive 5, the synchronization brake 14 or the service brake 13 can also be provided to increase the torque in the drive train in the dynamic (normal) operation of the differential system—i.e., here the differential drive 5 and the synchronization brake 14 or the service brake 13 act in the same torque direction, in which case a correspondingly high overall torque in the drive train can be achieved.

By actuating the synchronization brake 14, the drive shaft 2 is necessarily accelerated, whereby the torque that is available for this purpose is determined by the minimum from the braking force of the synchronization brake 14 acting on the drive shaft 2, on the one hand, and the breakdown torque of the prime mover 4, on the other hand. That is to say, in contrast to the starting options according to the state of the art, here, the multiple rated torque can be implemented as the starting torque from speed zero since the typical breakdown torque of a three-phase machine is roughly 2 to 3 times its rated torque. In principle, this starting method can also be used in, e.g., internal combustion engines, which is sometimes necessary since in the partial speed range, the latter can only generate a torque that is much smaller than their rated torque.

As a synchronization brake 14, for example, a disk brake (=mechanical brake) is used, with which the latter can also serve as a service and safety brake for the differential drive 5. Thus, the synchronization brake 14 can in principle also perform the function of the motor brake 13 that is shown in FIG. 1.

Alternatively, however, any type of brake can be used. In particular, so-called retarders are suggested here. First of all, the group of hydrodynamic retarders (=hydraulic brake) should be named here. For the most part, hydrodynamic retarders work with oil or water that if necessary is routed into a converter housing. The converter housing consists of two rotationally-symmetrical blade wheels that are opposite one another, and prior to this, a rotor that is connected to the drive train of the unit, and a stationary stator. The rotor accelerates the supplied oil, and the centrifugal force presses it to the outside. The shape of the rotor blades routes the oil into the stator that in this way induces a braking torque in the rotor and subsequently then also brakes the entire drive train. In an electrodynamic retarder (=electrical brake), e.g., an eddy-current brake, e.g., two steel disks (rotors) that are not magnetized are connected to the drive train. In between is the stator with electrical coils. When current is applied by activation of the retarder, magnetic fields are generated that are closed by the rotors. The magnetic fields in opposite directions then generate a braking action. The heat that is produced is released again by, e.g., internally-ventilated rotor disks.

An important advantage of a retarder as a service brake is its freedom from wear and tear and ease of control.

The system according to the invention can also be used to operate the prime mover 4 in phase-shifting operation. That is to say, the prime mover 4 can deliver or draw reactive current into or out of the grid 12 without the driven machine 1 being operated. This applies in particular to power plants. In this case, the prime mover 4 is only connected to the grid 12, without implementing the additional steps of the described starting process. This is done only when the driven machine 1 has to integrate the operation.

FIG. 3 shows another embodiment according to the invention of a differential system with a gear driving stage 16. This gear driving stage 16 makes it possible to match the speed range for the drive shaft 2 or for the driven machine 1 according to the transmission ratio of the gear driving stage 16. The use of a gear driving stage 16 is necessary and advantageous when the speed level that results based on the technical parameters of, e.g., an economical prime mover 4 and of an efficient differential system does not correspond to the required operating speed range of a driven machine 1. A resulting advantage is that if the gear driving stage 16 as shown is a spur wheel stage, the differential drive 5 can be positioned without an adjusting gear system 10 according to FIGS. 1 and 2 coaxially to the prime mover 4 on the side of the differential gear system 3 facing away from the prime mover.

In order to achieve a higher transmission ratio in the differential gear system 3 that is necessary due to the elimination of the adjusting gear system 10, it is suggested to use so-called stepped planets instead of simple planets. These stepped planets each consist of two gear wheels that are connected in a torque-proof manner with a different diameter and preferably different toothing geometry. The ring gear 8 then engages the smaller-diameter gear wheel of the stepped planet, and the sun wheel 9 engages the second gear wheel of the stepped planet. As an alternative, however, instead of the spur wheel stage depicted in FIG. 1, a planetary gearing stage can now also be produced as an adjusting gear system 10. Both the synchronization brake 14 and the clutch 15 can—depending on the desired speed/torque ratios—be positioned either in front of or behind the adjusting stage 10.

The connecting shaft 26 between the differential gear system 3 and the differential drive 5 is preferably an electrically nonconductive fiber composite shaft. If the connecting shaft 26 is an electrically conductive shaft, preferably an insulating element can then be installed between the differential gear system 3 (or, if present, the adjusting gear system 10) and the differential drive 5 in order to keep unwanted electrical current away from the differential gear system 3.

Thus, the differential system consists of a number of components that is as small as possible and, moreover, has an optimum overall efficiency. The motor brake 13 in the illustrated configuration also performs the function of the synchronization brake 14 from FIG. 2. The disadvantage of this embodiment compared to the one according to FIG. 2 is that the differential drive 5 must be designed for the starting process according to the invention for a higher speed, whereby the differential drive 5 at speeds above the governed speed range is preferably separated from the grid. Thus, speeds outside of the governed speed range need be only mechanically tolerated. In addition, to make matters worse, the transmission ratio of the differential gear system 3 must be higher than for the design according to FIG. 2, since here the adjusting gear system 10 is absent. In principle, however, the additional use of an adjusting gear system 10 is also possible for the variant according to FIG. 3, as a result of which the transmission ratio of the differential gear system 3 can be smaller. Moreover, a clutch 15 and a synchronization brake 14 can also be implemented between the second input of the differential gear system 3 or sun wheel 9 and the differential drive 5.

In principle, this embodiment can also be used for power plants, especially wind power plants and hydro-electric power stations, as the driven machine 1. In this case, compared to, e.g., a pump as the driven machine 1, the power flow direction is reversed, and the prime mover 4 works as a generator. If necessary, there can be one or more further gear stages between the gear driving stage 16 and the driven machine 1, which gear stages are then preferably designed as planetary gear stages.

Another advantage of this embodiment with gear driving stage 16 is that a coaxial hollow shaft 27 to the driven machine 1 can be easily implemented. By means of this hollow shaft 27, the turning driven machine 1 can be easily supplied electrically or hydraulically. Here, preferably rotational transmission 28 to the side of the gear driving stage facing away from the driven machine is applied. In principle, a mechanical rod can also be routed in the bushing 27 and thus by translational or rotary motion, e.g., the blades of a pump rotor can be mechanically adjusted.

If the differential system and the gear driving stage 16 are provided as so-called “stand-alone” variants, the drive shaft 2 and the prime mover 4 are preferably connected by means of a clutch 17, 18.

FIG. 4 shows the parameters of speed and power of a differential system, for example for a pump. The illustration shows power and speed values for a pump as a driven machine 1, a prime mover 4, and a differential drive 5, in each case plotted over the speed values of the drive shaft 2 (“pump speed”). The prime mover 4 is connected to the grid 12 and thus its speed (“motor speed”) is constant—in the illustrated example, roughly 1,500 μminute for a four-pole three-phase machine in a 50-Hz grid. The operating speed range for the drive shaft 2 ranges from 68% to 100%, whereby 100% is the chosen rated or maximum working point. According to the transmission ratio of the differential system, the speed of the differential drive 5 (“servo speed”) ranges from −2,000 l/min to 1,500 μmin. This means that the differential drive 5 is operated by generator (−) and motor (+). Since the maximum required power of the differential drive 5 in the generator (−) region (roughly 110 kW) is smaller than that in the motor (+) region (roughly 160 kW), the differential drive 5 in the generator (−) region can be operated in the so-called field weakening range, with which for the differential drive 5, a higher speed—but with reduced torque—can be implemented. Thus, the speed range for the driven machine 1 can be easily expanded.

Another possibility for expanding the speed range for the driven machine 1 features the so-called 87-Hz characteristic for the operation of the frequency converter 6. The principle here is the following: motors can typically be operated in star (400 V) or delta (230 V). If a motor is operated as usual with 400 V in a star connection, then the rated working point is reached at 50 Hz. This characteristic is set in the frequency converter. A motor can also be operated with 400 V in a delta connection, however, and the frequency converter can be parameterized such that it reaches 50 Hz at 230 V. In this way, the frequency converter reaches its rated voltage (400 V) only at 87 Hz (√3×50 Hz). Since the motor torque is constant up to the rated working point, a higher power is achieved with the 87-Hz characteristic. Here, however, it is to be considered that compared to the star connection, in the delta connection, the current is higher by √3. That is to say, the frequency converter must be sized to be stronger. Moreover, in the motor, due to the higher frequency, even higher losses arise for which the motor must be thermally designed. Ultimately, however, with the 87-Hz characteristic, a correspondingly (√3) higher speed range is attained with—in contrast to field weakening—a torque that is not reduced.

The “T” point in FIG. 4 marks the so-called “base speed” of the drive shaft 2, at which the speed of the differential drive 5 is equal to zero. Ideally, this “T” point is placed in a working range in which the unit is operated over large time intervals. At this operating point, the motor brake 13 can be activated, with which the differential drive 5 need not be operated, and subsequently, associated losses and wear and tear are avoided. In the motor (+) region of the characteristic diagram, the input is driven in parallel from the prime mover 4 and the differential drive 5. The sum of the two powers is the input power for the drive shaft 2 (“system power”)—minus the system losses that arise. In the generator (−) region, the prime mover 4 must compensate for the power of the differential drive 5 (“servo power”), as a result of which the overall system power (“system power”) is the input power of the prime mover 4 (“motor power”) minus the power of the differential drive 5. That is to say, in terms of efficiency, the motor (+) region is better. This matches extremely well the illustrated exemplary frequency distribution (“probability”) of the load distribution in continuous operation of the unit, which shows a large part of the duration of operation in the motor (+) region. As dictated by operation, however, an operation at lower pump speeds is also necessary, whereby here the proportional dwell time decreases greatly with decreasing pump speed.

In principle, it can be established that the closer the pump speed (“pump speed”) is to the base speed “T,” the smaller the power flow via the differential drive 5, and thus the overall system efficiency is also very high. Since, with increasing pump speed, the required input power also rises, however, compared to an input according to the state of the art, the required size of the prime mover 4 can be reduced by the size of the differential drive 5 by the parallel input of the prime mover 4 and of the differential drive 5.

As was already mentioned initially, according to German Utility Model DE 20 2012 101 708 U, the transmission ratio of the differential drive can be fixed at 1 using a differential blocking device. Thus, it is possible with the differential drive 5 to accelerate the complete drive train to the synchronous speed of the prime mover 4 and then to synchronize the latter with the grid. Subsequently, the differential drive 5 can be alternately switched off, and the prime mover 4 drives the driven machine 1 with synchronous speed alone. In addition, the differential drive 5 can drive the driven machine 1 parallel to the prime mover 4, with which a higher overall drive train power can be implemented. Thus, two steady-state operating points of the drive train can be implemented with the differential blocking device and the motor brake 13. In one especially economical embodiment, the differential drive is made lower-power such that with it, only the prime mover 4 is synchronized with the grid 12, or the differential blocking device. This can alternatively also be accomplished, however, by optional driving of the output or of the first input of the differential gear system 3.

If the prime mover 4 is to be synchronized only smoothly with the grid, the latter can be synchronized to the grid with a small frequency converter. Then, the second input is braked to speed zero by means of the synchronization brake 14 and thus the driven machine is accelerated. Since no differential drive 5 is provided in this simple embodiment, however, only a fixed operating speed can thus be achieved.

FIG. 5 shows another embodiment according to the invention of a differential system with a simplified differential drive. In this variant embodiment, the grid-side inverter 6 b is replaced by a simple rectifier 19. For the most part, the latter has a higher efficiency than an inverter 6 b and is also much more durable and economical. The sole limitation by the use of a rectifier 19 is that the differential drive 5 can only continue to be operated by motor (+).

If, in the reverse case, the differential system is operated only by generator (−), the motor-side inverter 6 a can be replaced by a rectifier 19 while maintaining the grid-side inverter 6 b.

FIG. 6 shows the parameters of speed and power following from FIG. 5 at the same operating speed range for the drive shaft 2 as in FIG. 4 (68%-100%). Due to the fact that the differential drive 5 continues only to be operated in the motor (+) region, the maximum power flow via the differential drive 5 is much greater than in the example shown above. At the rated working point, the required power of the differential drive 5 (“servo power”) reaches roughly 500 kW, which is 50% of the total drive power (“system power”). This results in that the frequency converter 6 a, 19 must also be dimensioned to be correspondingly large. The advantage of this variant is that the transmission ratio of the differential gear system 3 can be much smaller than for the variant according to FIG. 3, and thus when the system starts up according to the invention, the speed of the differential drive 5 that is the maximum that can be attained in doing so is smaller.

FIG. 7 shows another embodiment according to the invention of a differential system with a gear shifting stage. In the illustrated embodiment, the gear driving stage 16 is expanded by another gear driving stage 20 with a transmission ratio that is different from the gear driving stage 16. By means of a shifting device 21, it is possible to choose between the two gear driving stages, and thus an adjustable gear system 16, 20, 21 is obtained that can implement two speed ranges for the drive shaft 2. Alternatively, multiple shifting stages can also be implemented.

FIG. 8 shows the parameters of speed and power that follow from FIG. 7. In principle, the illustration contains two characteristic diagrams—each of which is similar to FIG. 6, but each with a smaller operating speed range for the driven machine 1. These characteristic diagrams are offset to one another by the two-stage adjustable gear system 16, 20, 21 with which at the same overall operating speed range for the pump (“pump speed” 68%-100%), a size for the differential drive 5 that is smaller in comparison to FIG. 6 is necessary. Moreover, in the characteristic diagram with lower system power, the differential drive 5 can be operated in the field weakening range, since here, the torque that is necessary for the differential system is in principle lower than its rated torque. Thus, the operating speed range in the characteristic diagram with the lower system power is larger than that for the second characteristic diagram. The two characteristic diagrams overlap one another preferably in the hysteresis region “H” in order to avoid frequent shifting between the characteristic diagrams. The hysteresis region “H,” however, burdens a differential system that is still smaller in terms of power, and can, if no overlapping of the two characteristic diagrams is necessary, also be smaller or be omitted entirely.

FIG. 9 shows another embodiment according to the invention of a differential system with a reduced speed range. In principle, the drive train is built the same as already shown in FIG. 5. In the power system 29 of the driven machine 1 (e.g., a pump, a compressor, or a fan), a choke 22 is integrated following the latter. Thus, the amount delivered by the driven machine 1 can be choked, without for this purpose reducing the speed of the driven machine 1. This choke 22 is conventionally used in drives that are not variable-speed drives in order to regulate/control the delivered amount. The choke 22 can have the most varied embodiments, whereby a simple flap constitutes a conventional variant.

In principle, for the variant according to FIG. 9, the additional use of an adjusting gear system 10 is also possible. Moreover, a clutch 15 and a synchronization brake 14 can also be implemented between the second input or the sun wheel 9 and the differential drive 5. Furthermore, the gear driving stage 16 is also not critically necessary.

In order to make the size of the differential drive 5 and of the frequency converter 6 a, 19 as small as possible, instead of the rectifier 19, a grid inverter 6 b can also be used, and thus the system can be operated by motor (+) and generator (−), as a result of which the size of the differential drive 5 is decisively reduced. Thus, the base speed (“T” point) moves into the middle of the operating speed range, in which the differential drive 5 can be braked, and thus the differential system can be operated especially efficiently. Small variations of the amounts delivered (such as, for example, in pumps) or variations that are necessary based on operation can be compensated/regulated here with the choke 22.

One possibility for expanding the operating speed range for the driven machine 1 is offered, as already described in FIG. 4, by the field weakening range or the so-called 87-Hz characteristic for the operation of the differential drive 5 and of the frequency converter 6 a, 6 b or 19.

FIG. 10 shows the parameters of speed and power that follow from FIG. 9. The chosen operating range of the differential system thus moves into a region with a high operating frequency distribution (“probability”). As soon as the differential drive 5 reaches the base speed (“T” point) as the pump speed decreases, the latter is preferably braked or stopped. A lower delivery rate that is necessary for reasons of operating technology is implemented by activation (regulation/control) of the choke 22. The speeds of the differential system remain essentially constant here.

FIG. 11 shows the possible parameters of speed and power for a so-called pump turbine that follow from FIG. 9 (a choke 22 can be omitted here). In this application, the system is preferably operated by motor (+) above the base speed (“T” point) and by generator (−) below the base speed. Here, in generator operation, the prime mover 4 works as a generator that is connected to the grid 12. The differential drive (5) continues to be operated by motor (+) at a driven machine speed below the base speed due to the power flow reversal. This yields an electrically simple system that can be implemented without a grid-side inverter. Since, however, the power flows from the generator (4) and the differential drive (5) are opposite below the base speed and thus the system efficiency is poorer than in purely motor operation—if possible in terms of operating technology—the procedure can be performed at a fixed speed, i.e., preferably with the differential drive 5 stopped, entirely or partially in this mode of operation. Ideally, the operating points are then placed such that the pump turbine at the base speed (“T”) has an optimum efficiency for the turbine operating mode.

FIG. 12 shows another embodiment according to the invention of a differential system for an internal combustion engine 23 as the prime mover. Since the internal combustion engine 23 is not connected to an electrical grid, the required energy for the differential drive 5 is taken from the first input of the differential gear system 3 or is supplied to it. Here, two motor-side inverters 6 a are connected by means of a d.c. intermediate circuit and drive another differential drive 25. The latter is connected to the first input of the differential gear system 3 by means of the adjusting gear system 24. The adjusting gear system 24 that is shown as single-stage can also be multi-stage if necessary. Thus, the energy circuit is closed, and the system can be operated by both generator (−) and also motor (+), more or less independently of the grid. If the design speeds of the internal combustion engine 23 and of the differential drive 25 go well together, the adjusting gear system 24 can be omitted, and the differential drive 25 is directly (by means of a clutch) coupled to the internal combustion engine 23.

Ideally, the electrical part of the differential system, consisting of the differential drives 5 and 25 and the two inverters 6 a, is also connected to a grid. Thus, for example, the starting scenarios that are described for FIGS. 1 to 3 can be easily implemented and/or (as is conventional, e.g., in ship propulsions) a power grid can be supplied. Moreover, the integration of a shifting stage according to FIG. 7 is also possible.

Instead of the differential drives 5 and 25 and the two inverters 6 a, a hydrostatic actuating gear can also be used. In doing so, the differential drives 5 and 25 are replaced by a hydrostatic pump/motor combination, which is connected to a pressure line and which both can preferably be adjusted in flow volume. Thus, as in the case of a variable-speed electrical differential drive, the speeds can be regulated. This also applies for applications with an electrical machine as the prime mover (4).

The major advantages that arise for the operation of an internal combustion engine 23 in combination with a differential system are, on the one hand, the attainable high starting torque according to the invention and the fact that the internal combustion engine can be run in an efficiency-optimum range as soon as the differential system takes over the speed matching for the driven machine 1. Because, in contrast to a grid-coupled three-phase machine, an internal combustion engine can be operated at variable speed, the range of possibilities for expanding/varying the characteristic diagrams of the system is wide.

FIG. 13 shows a regulating system for damping drive train vibrations. The torque on the differential drive 5 is proportional to the torque in the entire drive train, as a result of which torque regulation/control and also drive train damping by the differential drive 5 become possible. Drive train damping is defined here as the dedicated correction of mostly rotational drive train vibrations (driven machine 1, drive shaft 2, differential gear system 3, prime mover 4, and differential drive 5), which can occur transiently or constantly and which lead to unwanted loads in the entire drive train or in parts of it. This is achieved by modulation of the torque and/or of the speed of the differential drive 5 with vibrations of the same frequency.

Such unwanted drive train vibrations or transient drive train loads can arise either by loads acting from the outside on the driven machine 1, in the drive shaft 2, the differential gear system 3, and the differential drive 5 themselves or by the prime mover 4 and are typically apparent in the speed behavior or torque behavior of the drive train.

Preferably, the latter can be detected by measurements of speed and/or vibration in the drive train or by current measurements on the prime mover 4 and/or on the differential drive 5. Direct detection of torque is likewise possible, but for the most part is difficult to accomplish. The type of detection, however, always depends ultimately on at which location in the drive train the damping is to take place and whether couplings can be used.

If drive train vibrations are caused by, e.g., a typical operating behavior on the driven machine 1, and if they are to be compensated in their action on the prime mover 4, they can be reduced or extinguished by impressing torque vibrations in phase opposition on the differential drive 5. This is the case, e.g., in compressors in which design-specific vibration excitations that correlate strongly with the piston position occur when the piston rod is revolving. Since the respective vibration excitation always occurs at the same piston position, it is sufficient to know the peripheral position or rotary position, e.g., by measurement, in order to be able to compensate for this. The knowledge of this vibration excitation allows selective compensation of individual or multiple vibrations at the same time. This is preferably achieved by detecting the position of the piston rod or by one of the above-cited methods. The necessary synchronous torque/speed matching in phase opposition is implemented by conventional methods of signal processing preferably with oscillators and notch-filter algorithms that simulate and evaluate the measured vibration excitation with the correct frequencies. Incorporated into an oppositely-coupled system, the necessary amplitudes and phase angles are thus automatically established for the vibrations that have been generated for compensation and with which then the actuator on the differential drive 5 is activated.

As is shown by way of example in FIG. 13, a comparator circuit 30 is supplied with a target constant speed n₄ of the prime mover, on the one hand, and the speed n₂ of the drive shaft 2. A regulating system 31 controls the differential drive 5 using the desired speed n_(5desired) that has been determined therefrom and the actual speed n₅ of the input shaft of the differential drive 5 via the frequency converter 6 such that vibrations of the prime mover 4 are damped as well as possible or as desired. The drive train damping that is described with reference to FIG. 13 can also be used independently of all other embodiments that were described above. 

1. Method for starting a drive train with a drive shaft (2), a prime mover (4), and with a differential gear system (3) with three inputs and outputs, whereby one output is connected to the drive shaft (2), a first input is connected to the prime mover (4), and a second input is connected to a differential drive (5), characterized in that the prime mover (4) is started up from a speed of zero or approximately zero, while an external braking torque acts on the drive shaft (2), and in that in an acceleration phase of the drive shaft (2), the second input is braked.
 2. Method according to claim 1, wherein as soon as the prime mover (4) has reached an operating speed, the second input is braked until the speed of the second input has reached a speed in which the speed of the differential drive (5) lies in its governed speed range.
 3. Method according to claim 2, wherein the second input of the differential drive (5) is first decoupled and after reaching a speed at which the speed of the differential drive (5) lies in its governed speed range, it is coupled to the differential drive (5).
 4. Method according to claim 1, wherein in an acceleration phase of the drive shaft (2), the second input is braked, so that the speed of the differential drive (5) remains in its governed speed range.
 5. Method according to claim 4, wherein the braking power is generated by the differential drive (5).
 6. Method according to claim 1, wherein the braking power is generated by a mechanical, electrical or hydraulic brake (13, 14) that is connected to the differential drive (5).
 7. Method according to claim 1, wherein the second input is directly braked mechanically, electrically, or hydraulically.
 8. Method according to claim 1, wherein the prime mover (4) is an electrical machine that is connected to a power grid (12), which machine is brought to synchronous speed connected to the power grid (12) with a star/delta circuit, and then is synchronized with the power grid (12).
 9. Method according to claim 1, wherein the prime mover (4) is an electrical machine that is connected to a power grid (12), which first is brought to an at least approximate synchronous speed with an additional system and then is synchronized with the power grid (12).
 10. Method according to claim 1, wherein the prime mover (4) is accelerated with the differential drive (5).
 11. Method according to claim 10, wherein the prime mover (4) is an electrical machine that is connected to a power grid (12) and is connected to the power grid (12) after the acceleration.
 12. Method according to claim 1, wherein the prime mover (4) is an electrical machine that is connected to a power grid (12), which machine accelerates by means of a frequency converter (6) of the differential drive (5) and is synchronized with the power grid (12), wherein the electrical machine (4) is then connected to the power grid (12) and wherein finally the prime mover (4) is separated from the frequency converter (6) and the latter is connected again to the differential drive (5).
 13. Input for implementing a method according to claim 1 with a drive shaft (2), a prime mover (4) and with a differential gear system (3) with three inputs or outputs, whereby one output is connected to the drive shaft (2), one input is connected to the prime mover (4), and a second input is connected to a differential drive (5), wherein the second input is connected to a brake (13).
 14. Input according to claim 13, wherein the prime mover (4) is a three-phase machine that is connected to a power grid (12).
 15. Input according to claim 13, wherein the prime mover is an internal combustion engine.
 16. Input according to claim 13, wherein the differential drive (5) is a three-phase machine.
 17. Input according to claim 13, wherein the differential drive is a hydraulic pump/motor.
 18. Input according to claim 13, wherein the differential drive (5) is connected to the second input via an adjusting gear system stage (10).
 19. Input according to claim 13, wherein the brake (13) is a disk brake.
 20. Input according to claim 13, wherein the brake (13) is a retarder.
 21. Input according to claim 13, wherein the differential drive (5) is connected to the second input via a clutch (15).
 22. Input according to claim 13, wherein the differential drive (5) and the synchronization brake (14) or the service brake (13) act in the same torque direction. 